When is dusk dusky enough?

I shoot a lot of architectural photos at dusk because many buildings look much more dramatic at that time of day. As I said in this post, one doesn't do this type of shot at night, even though many people refer to them as "night shots". At night the sky is black and uninteresting. At dusk, the sky is a marvelous shade of blue, and when you balance for the greenish fluorescent and sodium lights, the sky takes on a lot of magenta and the color gets even better. I'm doing this post to point out that just because you think it's dusk, it's probably not late enough. When you think it's starting to get just right, take a meter reading on the sky and do a test shot. You'll probably see something like this first photo.

The second photo was taken about 20 minutes later when it was really getting pretty dark and the sky is truly balancing with the lighting in the building. This requires a tripod because the exposure was f8 at about 3 to 5 seconds.

The point is you really have to stand there and wait for it and do a lot of test shots. Doing a good dusk photo usually means at least an hour of standing around for me, but when it gets right, there are only a few minutes to do any variations needed.